An IP Multimedia Subsystem (IMS) is a system which supports Internet Protocol (IP) multimedia services. The IMS has been proposed by the 3G mobile network with support of the 3rd Generation Partnership Project (3GPP). The IMS is related to services and applications and enables telecommunication companies, mobile network operators, and other service providers to provide a rich variety of multimedia services via a next generation packet switch network. At present, the IMS has been extended to cable networks and serves as a key technique in fixed networks and mobile networks.
The IMS is based on a Session Initiation Protocol (SIP). The SIP is a key technique in the communication control of the Next Generation Network (NGN) based on the IP. Referring to FIG. 1, an IP network telephone using the SIP under a private network is taken as an example to describe the structure of a conventional IMS system networking. A User Equipment (UE) of the IP network telephone under the private network is registered on a Proxy-Call Session Control Function (P_CSCF) via a Network Address Translation (NAT), an Access Border Gateway Function (A_BGF), and other apparatuses. A registration path acquired by the UE after a successful registration is: UE-NAT-A_BGF-P_CSCF. SIP signals are forwarded by the registration path.
A plurality of entries for stream forwarding, i.e. session tables, is stored in such stream forwarding apparatuses as the NAT and the A_BGF. When a message is forwarded, a session table is obtained first by searching according to a key value which includes a source IP address, a destination IP address, a source port, a destination port, and a protocol number. After the session table is found, i.e. the session table is hit, an outbound interface index is obtained by searching in a fast Forwarding Information Base (FIB) entry according to the destination IP address in the session table, a next hop information is obtained by searching in a routing table, and a destination Media Access Control (MAC) address is obtained by searching in an Address Resolution Protocol (ARP) entry. The message is forwarded according to the destination IP address, the outbound interface index, the next hop information, and the destination MAC address.
An aging_time and a time_stamp are included in the session table. The aging_time denotes a time when that the session table is aged. The time_stamp is a time counter and counts from 0 when the session table is created. When the value of the time_stamp is greater than that of the aging_time, i.e., the entry has been aged, the session table is erased.
Since the NAT, the A_BGF, and the like all are stream forwarding apparatuses, the registration path of the UE is unique. Throughout the duration of the user's conversation, the session tables in the A_BGF and the NAT have to be kept alive to ensure that the registration path is unblocked. In the IMS system, since the P_CSCF periodically sends a command for keeping session table alive to the A_BGF, the session table in the A_BGF is kept alive. Throughout the duration of the user's conversation, the session table in the NAT apparatus has to be kept alive to ensure that the registration path is unblocked.
At present, when the IMS technique is used, there are two methods for keeping the session table alive in the NAT apparatus. An IP network telephone under a private network is taken as an example.
A UE of the IP network telephone under the private network sends a re-registration message to a P_CSCF via the NAT apparatus. After a session table in the NAT apparatus is hit by the re-registration message, the NAT apparatus refreshes the time_stamp in the session table to prevent the session table from being aged during the conversation, thus keeping the registration path unblocked.
However, the time when the UE of the IP network telephone sends the re-registration message is set by the manufacturer of the UE and varies with the manufacturer who configures the time. In addition, the time when a product sends the re-registration varies with the performance of the product. Since the time when the UE of the IP network telephone sends the re-registration message cannot be set flexibly, it is quite possible that the session table of the NAT apparatus has already been aged before the UE sends such a message.
Therefore, it is necessary to prolong the life of the session table, i.e., make the aging_time of the session table in the NAT apparatus longer than the user's conversation duration, thus keeping the registration path unblocked.
However, on the one hand, since there is a considerable difference between the conversation durations of the users, it is difficult to determine the value of the aging_time, and it cannot be ensured that the prolonged aging_time in the session table is definitely longer than the conversation durations of the users. On the other hand, over-prolonging the aging_time in a session table will inevitably affect the performance of the NAT apparatus.